The Steam for Linux beta has been rolled out like mince pie pastry for everyone to enjoy.
Just install and register to use Steam and you're in.
The list of games available to Linux users of Steam is growing, too. Killing Floor is there as is The Book of Unwritten Tales and Amnesia. There are 39 games available there now.
Presumably the open beta means a full launch for Steam on Linux isn't too far off. Keep an eye on the Steam Linux community site for further developments.
Now all we need is a picture of Gabe Newell wearing a Santa hat.